Polarimeters are used to view and quantitatively measure stress in glass and other photoelastic materials. When isotropic, photoelastic materials, such as glass and some plastics, are stressed, they become anisotropic. The anisotropy within the material makes the stressed areas birefringent. With a polarimeter, one can determine the degree of anisotropy by observing the degree birefringence. This can then be related qualitatively and quantitatively to the amount stress within the material.
Polarimeters generally consist of a light source, a first polarizer, a ¼ wave plate and a second rotatable polarizer. In order to observe the birefringence, a sample of the material is placed between the first polarizer and the ¼ wave plate. Any stress with a principal axis 45° to either polarizer will appear as a bright area in a dark background. The intensity of the bright area is proportional to the amount of stress. By rotating the second polarizer, the birefringence can be cancelled causing the bright area to become extinct (dark). The degrees of rotation can be converted to a measurement of the phase shift in the light (birefringence) by multiplying the dominant wavelength of light and dividing by 180°. This results in a measurement of the birefringence that can be converted into the stress intensity by dividing by the stress optical coefficient and the optical path length.
A common light source for a polarimeter is an incandescent lamp. Although this produces a smooth spectrum of white light, nearly 85% of the energy input to the lamp is converted to heat. This excessive heat causes damages to the polarizing film of the first polarizer which is adjacent to the lamp. Over a relatively short period, the damage to the film diminishes the ability to measure stress.
A second disadvantage with incandescent lamps and other white light sources is chromatic abberations. As the birefringence in the material increases, the longer, red wavelengths will resolve at different positions (degrees of rotation) of the second polarizer from the shorter, blue wavelengths. This creates two overlapping images which makes it increasingly difficult to properly resolve birefringence of >60 nm. The error in the measurement increases with the degree of birefringence. As a result, what should be a relatively simple and repeatable measurement becomes one requiring a great deal of skill and experience and exhibiting a high degree of variation from one operator to another.
Some polarimeters use compact fluorescent lamps to diminish the damage to the polarizing film from heat. However, this greatly increases chance for chromatic abberations. When a polarimeter lamp fails, it must be replaced with a reasonably exact duplicate otherwise the difference in the emission spectrum will shift the calibration of the polarimeter. In the case of fluorescent lamps, it is easy to make a mistake since the emission spectrum of a fluorescent lamp is a function of the phosphors used in its manufacture, e.g., there are several “colors” of white for commercial fluorescent lamps. Likely, the operator will be unaware of the color shift of the lamp and therefore not able to compensate for it.
Thus, it would be an advantage to have a light source for a polarimeter which would overcome these difficulties.